Both environmental and genetic factors involved in the disturbance of cholesterol metabolism have been suggested as risk factors for the development of Alzheimer's disease (AD). Accumulation of cholesterol has been observed in affected brain areas from AD patients and animals, and it is associated with region-specific loss of synapses. Elevated brain cholesterol causes cognitive deficits, amyloid-? (A?) production and aggregation, and tau pathology. Despite these observations, the mechanisms that govern brain cholesterol homeostasis and influence on neurons under AD-related pathological conditions remain elusive. In particular, the field lacks knowledge on the factors that are involved in the signaling pathways of neuronal cholesterol metabolism, related to the initiation and development of AD pathology. ATAD3A belongs to a new family of eukaryotic mitochondrial AAA-ATPases. ATAD3A regulates cholesterol homeostasis and trafficking via an unknown mechanism at the mitochondria-associated ER membrane (MAM), a specialized subdomain of the ER that has the features of a lipid raft and is rich in cholesterol and sphingomyelin. Our recent work demonstrated that ATAD3A, via pathological dimerization, showed a gain-of-function that caused neurodegeneration in Huntington's disease. We further observed an enhancement of ATAD3A oligomerization in AD neuronal culture, in AD transgenic mouse brains and in AD patient postmortem hippocampus, suggesting an aberrant activity of ATAD3A in the pathogenesis of AD. We developed a novel peptide inhibitor DA1 that binds to ATAD3A to block ATAD3A dimerization. Notably, sustained treatment with DA1 reduced APP level and amyloid load, attenuated neuro-inflammation and improved short-term spatial memory in 5XFAD transgenic mice. Further, our proteomic analysis suggests that blocking ATAD3A oligomerization by DA1 treatment mainly influenced the cholesterol metabolic pathway in AD mouse brains. The treatment in AD transgenic mice improved brain cholesterol turnover and did not affect brain phospholipids levels. Moreover, we showed that DA1 treatment reduced cholesterol burden and oxidative stress in neuronal cells stably expressing APP wt or mutant. These findings highlight ATAD3A oligomerization as a previously unidentified mechanism underlying brain cholesterol disturbance and neurodegeneration in AD. Our central hypothesis is that ATAD3A oligomerization mediates amyloid pathology, leading to neurodegeneration, by impairment of brain cholesterol metabolism. The overall goal of this application is to understand ATAD3A aberrant oligomerization-mediated neuropathology in AD, and to reveal a novel therapeutic target for AD. In Aim 1, we will determine the impact of ATAD3A oligomerization on brain cholesterol homeostasis, AD pathology and behavioral deficits in AD mice. In Aim 2, we will determine whether haplosufficiency of ATAD3A in AD mice restores brain cholesterol homeostasis and reduces AD pathology. In Aim 3, we will dissect the mechanistic links between ATAD3A oligomerization and disturbance in brain cholesterol homeostasis in AD.